Scuba tank problem

[SkydiverChick]

We were all standing on the back deck of the boat fishing and all the sudden a loud noise. It's air coming out of one of the tanks I rented. The air was turned off to begin with and only stopped when it ran out of air. Why would this happen? I'm new to diving so nothing like this has ever happened. It kind of freaked me out because what if I was underwater using this tank when it happened?

 

[tfsails]

The burst disc burst. Sounds to me like the tank got hot and overpressurized, so the burst disc let go. Don't worry--not your fault. Maybe the tank was overfilled. Maybe it was a black tank and sat out in the sun while the pressure built up inside it and it overcame the burst disc.

This is, believe it or not, a good thing; had the disc not let go, the tank itself may have ruptured, and you (and all around you) would have had a VERY BAD DAY. Every tank valve has a burst disc installed. At least, it's supposed to.This is a safety feature designed into the valve to prevent tank overpressurization. It should be rated at 5/3 the tank's working pressure. If a tank has a 3000psi working pressure, it should have a 5000psi burst disc installed in it.

It probably would not have ruptured underwater. Key word here: probably. Had it ruptured, you surface RIGHT NOW or you drown. It's that simple.

 

[mike_s]

the burst disc was most likely corroded.

when that happens, they weaken.



an AL80 uses a 3000psi typical fill, unless you have a different rated tank. The burst disk for 3000psi is typically a 5000psi burst disc. It's rated at 5/3rds the rated tank pressure. (although you can get a 4000psi burst disc for a 3000psi valve).

This wasn't for sitting in the sun. It' was most likely corrosion. For a tank to sit in the sun and get hot enough to get to 5000psi, it'd have to get up to 400 degrees f.

 

[Delta74]

Was a Burst disc that let go, its a built in safety device to prevent high pressure mishap's, since it is usually considered to be good practice to self check any tanks you rent or pick up, what was the pressure on it when you picked it up?and what kind of tank AL or steel?
let me guess you had it sitting under the hot sun after being moved in a hot trunk right? while it should not have let go, it could have been a bad disc that should have been replaced when it was serviced, or the tank was already filled well over service pressure then sun baked till the pressure built up to crazy levels. I am sure some of the others can guess assuming standard rental of a Al 80, 3000 PSI normal fill, pressure was guessing around 3600 to pop the disc.

 

[DandyDon]

Damn! :shocked2: Where did this happen? That's what a burst disc if for, but it's so rare! How long did it take to empty anyway...?

The burst disc burst. Sounds to me like the tank got hot and overpressurized, so the burst disc let go. Don't worry--not your fault. Maybe the tank was overfilled. Maybe it was a black tank and sat out in the sun while the pressure built up inside it and it overcame the burst disc.

This is, believe it or not, a good thing; had the disc not let go, the tank itself may have ruptured, and you (and all around you) would have had a VERY BAD DAY. Every tank valve has a burst disc installed. At least, it's supposed to.This is a safety feature designed into the valve to prevent tank overpressurization. It should be rated at 5/3 the tank's working pressure. If a tank has a 3000psi working pressure, it should have a 5000psi burst disc installed in it.

Even rarer.

It probably would not have ruptured underwater. Key word here: probably. Had it ruptured, you surface RIGHT NOW or you drown. It's that simple.

I think most would surface quickly or seek buddy's air for slower surfacing. Can't imagine anyone staying to see if they drown or not.

 

[OnceLoyal]

Freaky. We had one burst last time I was in Dominican. Filled to 3100psi, burst disc was fatigued and I assume the pressure went up from the sun and blew it out!!

 

[DA Aquamaster]

Saying that if it happens you surface right now or you drown is being way too melodramatic.

Obviously, from observing it on a boat you now know how long it took to empty on the surface, and it would take about half that long to dump a tank that was half full underwater as it is a fairly linear equation. That is more than enough time time to do a swimming ascent (assuming you are not in an overhead environment) or to connect with a buddy and share gas.

More importantly, it would be very rare for the failure to occur underwater after the pressure has dropped significantly from the fill pressure. The vast majority of burst disc failures occur during the fill, and most of the rest occur while the tank is sitting around with a full fill. It is rare to have a burst disc rupture during the dive.

A burst disc is designed to rupture at between 90% and 100% of the tank's test pressure. For a 3AL or 3AA tank, the test pressure is 5/3rds of the service pressure stamped on the tank. So for a 3000 psi 3AL tank, the test pressure is 5000 psi and the burst disc must rupture at between 4500 and 5000 psi.

What this means is that unless the burst disc is corroded, it is not going to fail at anywhere near the normal service pressure, even if the tank is painted black and is sitting in the sun in the middle of the sahara.

Burst discs should be, at a minimum, changed every time the tank itself is hydro tested. That ensures they get replaced every 5 years. Rinsing the tank and valve thoroughly after each salt water dive will hep prevent corrosion.

With tanks that are frequently used in salt water (rental tanks, etc) replacing the burst disc every couple years is a good idea. Tracking rental tanks is not hard in this regard - if the serial number ends in an odd digit, change it at the VIP in odd number years, if the serial number ends in an even digit, change it at the VIP in every even numbered year.

Consider it to be something that should be done on a regular interval whether it needs it or not - like getting your reserve chute repacked every 180 days.

 

[Drew Sailbum]

I am I the only one who does not vote for a burst disc? It could be an o-ring.

If the valve was recently pulled (as would be done for a visual inspection of the cylinder) and not tightened down sufficiently when re-installed then the o-ring at the neck seal could extrude. It makes a loud popping sound when it happens and there is no way to stop the cylinder from emptying.

It could be a failed o-ring in the valve itself. In this case you can feel the air coming out on the side of the valve opposite the burst disc. This will drain the cylinder much slower.

Either option can drain the cylinder with the valve in the off position. Neither requires a serious overfill of the cylinder or an early release of a burst disc.

 

[redacted]

Sure, burst disk, neck o-ring, seat, or even a cracked tank could cause such a leak. While most scuba gear is quite reliable, SH and you need to be aware of possible failures and prepared to handle them. Most failures are more of an annoyance that cause the early (but safe) termination of a dive but you also need to be prepared for the very rapid loss of gas. If you are adequately prepared for such a failure, you should still be able to safely terminate your dive.

 

[DA Aquamaster]

I am I the only one who does not vote for a burst disc? It could be an o-ring.

If the valve was recently pulled (as would be done for a visual inspection of the cylinder) and not tightened down sufficiently when re-installed then the o-ring at the neck seal could extrude. It makes a loud popping sound when it happens and there is no way to stop the cylinder from emptying.

It could be a failed o-ring in the valve itself. In this case you can feel the air coming out on the side of the valve opposite the burst disc. This will drain the cylinder much slower.

Either option can drain the cylinder with the valve in the off position. Neither requires a serious overfill of the cylinder or an early release of a burst disc.

Improper tightening of the valve requires a much mre direct failure to act properly on the part of the person installing the valve, while not changing an old burst disc is a much more likely act of ommission to occur.

As for an o-ring inside the alve failing, unless it is an older valve with a stem gauge in the valve (and those are very rare today) there are no other o-rings in the valve that would cause a tank to leak with the seat closed - any o-rings in the valve are downstream of the seat and you could in fact remove them/replace them with the valve closed and not experience a leak.

So, a leaking tank neck o-ring is not outside the realm of possibility, but it is not the first suspect in this type of scenario.